198 
SIR J. F. W. HERSCHEL ON THE ACTION OF THE RAYS 
the colour of the American Marigold*, causes its tint to pass rapidly in sunshine 
from brown to green, after which continued exposure produces no further change. 
The yellow colour of fresh bees’-wax and of palm-oil, are also, I doubt not, referable 
to the same, or a nearly similar colouring matter, both being very speedily bleached 
by exposure to light. 
193. Viola odorata . — Chemists are familiar with the colour of this flower as a test of 
acids and alkalies, for which, however, it seems by no means better adapted than many 
others ; less so, indeed, than that of the Viola tricolor, the common purple Iris, and 
many others which might be named. It offers, in fact, another, and rather a striking 
instance of the simultaneous existence of two colouring ingredients in the same 
flower, comporting themselves differently, not only in regard to light but to chemical 
agents. Extracted with alcohol, the juice of the violet is of a rich blue colour, which 
it imparts in high perfection to paper. Exposed to sunshine, a portion of this colour 
gives way pretty readily, but a residual blue, rather inclining to greenish, resists obs- 
tinately, and requires a very much longer exposure (for whole weeks indeed) for its 
destruction, which is not even then complete. Photographic impressions, therefore, 
taken on this paper, though very pretty, are exceedingly tedious in their preparation, 
if we would have the lights sharply made out. 
194. The residual tint thus outstanding, after long exposure, is turned, not green, 
but yellow, by alkalies ; or, if greenish at first, a very few hours suffice for the destruc- 
tion of the slight remnant of blue, and the consequent appearance of the yellow co- 
lour. Reasoning on this fact, as well as on the action of light above mentioned, it 
seems highly probable that the tincture in question holds in solution two distinct 
colouring principles, of which the one (greatly preponderant in quantity) is destruc- 
tible by light, and either destroyed or turned green by alkalies ; the other, indestruc- 
tible by light, and either naturally yellow in colour or changeable into yellow by al- 
kaline agency. 
195. This view of the composite nature of the colour in question receives corrobo- 
ration from the habitudes of the alcoholic tincture above mentioned, when rendered 
green by admixture of carbonate of soda. On making this addition it becomes evi- 
dent that a large amount of colour has been destroyed ; the green tint imparted by 
it to paper being far less intense than might be expected from the intensity of the 
original hue, and from the trifling dilution caused by the small quantity of alkaline 
liquid required to effect the change. What remains is a fine green ; but when ex- 
posed to light, the blue constituent alone of that green is destroyed, and a residual 
tint of pure yellow, which is very indestructible by light, is left. Exposure of a slip 
of such paper to the spectrum proves this change to be operated almost wholly by 
rays less refrangible than the fiducial yellow. A slight discoloration is perceived in 
the indigo-blue rays (at about -}- 30), but the green appears quite inactive. 
196. In the case of the purple Iris mentioned above, when turned green by the 
* French Marigold, Tagetes Patula. 
